25.0 mL of 0.10 M acetic acid is titrated with 0.10 M NaOH ( $K_{a} = 1.8 \times 1 0^{- 5}$ ). Indicator ranges: methyl orange 3.1-4.4; methyl red 4.2-6.3; bromothymol blue 6.0-7.6; phenolphthalein 8.3-10.0. The best indicator for the end point is

Answer: D

At equivalence, 2.5 mmol of acetate is in 50 mL, so

c = 0.050

M and pH

= 7 + \frac{1}{2} (p K_{a} + lo g c) = 7 + \frac{1}{2} (4.745 - 1.301) = 8.72

. The indicator whose range contains this value is phenolphthalein.